Environmental barrier coating for silicon-containing substrates and process therefor

ABSTRACT

A protective coating for use on a silicon-containing substrate, and deposition methods therefor. The coating has a barium-strontium-aluminosilicate (BSAS) composition that is less susceptible to degradation by volatilization and in corrosive environments as a result of having at least an outer surface region that consists essentially of one or more stoichiometric crystalline phases of BSAS and is substantially free of a nonstoichiometric second crystalline phase of BSAS that contains a substoichiometric amount of silica. The coating can be produced by carrying out deposition and heat treatment steps that result in the entire coating or just the outer surface region of the coating consisting essentially of the stoichiometric celsian phase.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a division patent application of U.S. patent application Ser.No. 10/709,288, filed Apr. 27, 2004, now U.S. Pat. No. 7,341,797.

BACKGROUND OF THE INVENTION

This invention relates to coating systems suitable for protectingcomponents exposed to high-temperature environments, such as the hot gasflow path through a gas turbine engine. More particularly, thisinvention is directed to a coating composition that exhibits improvedhigh temperature stability when used to protect a silicon-containingsubstrate.

Higher operating temperatures for gas turbine engines are continuouslysought in order to increase their efficiency. While nickel, cobalt andiron-base superalloys have found wide use for components throughout gasturbine engines, alternative materials have been proposed. Inparticular, silicon-based non-oxide ceramics, most notably with siliconcarbide (SiC) as a matrix and/or as a reinforcing material, arecandidates for high temperature applications, such as combustor liners,vanes, shrouds, airfoils, and other hot section components of gasturbine engines. Components in many of these applications are in contactwith highly corrosive and oxidative environments. It has been determinedthat Si-based ceramics lose mass and recede at high temperatures inwater-containing environments because of the formation of volatilesilicon hydroxide (Si(OH)₄). The recession rate due to volatilization orcorrosion can be sufficiently high to require an external coating withhigh resistance to such environments.

Stability is a critical requirement of a coating system for a Si-basedmaterial in high temperature environments containing water vapors. Otherimportant properties for the coating material include low thermalconductivity, a coefficient of thermal expansion (CTE) compatible withthe Si-based ceramic material, low permeability to oxidants, andchemical compatibility with the Si-based material and a silica scalethat forms from oxidation. As such, protective coatings for gas turbineengine components formed of Si-based materials have been termedenvironmental barrier coatings (EBC).

Barium-strontium-aluminosilicates (BSAS; (Ba_(1-x)Sr_(x))O—Al₂O₃—SiO₂)and other alkaline earth aluminosilicates have been proposed asprotective coatings for Si-based materials in view of their excellentenvironmental protection properties and low thermal conductivity. Forexample, U.S. Pat. Nos. 6,254,935, 6,352,790, 6,365,288, 6,387,456, and6,410,148 to Eaton et al. disclose the use of BSAS and alkaline earthaluminosilicates as outer protective coatings for Si-based substrates.Of these, all but U.S. Pat. No. 6,352,790 disclose stoichiometric BSAS(molar ratio: 0.75BaO.0.25SrO.Al₂O₃.2SiO₂; molar percent:18.75BaO.6.25SrO.25Al₂O₃.50SiO₂) as the preferred alkaline earthaluminosilicate composition, with layers of silicon and mullite(3Al₂O₃.2SiO₂) employed as bond coats. The BSAS coatings are typicallyproduced by air plasma spraying (APS) followed by heat treatment tocontain at least 50% of the celsian crystallographic structurecorresponding to stoichiometric BSAS. U.S. Pat. Nos. 6,254,935 and6,365,288 further teach that the coating contains a crystallinestructure of at least 80% by volume composed of crystalline celsian andhexacelsian phases, which differ crystallographically but have the samestoichiometric BSAS chemistry.

Notwithstanding the above-noted advances, further improvements incoating life are required. In particular, longer exposures attemperatures sustained in the combustion environment of a gas turbineengine (e.g., above 2300° F. (about 1260° C.) combined with highpressure steam and high gas velocities) have resulted in thevolatilization of existing BSAS materials, causing coating recessionthat ultimately leads to degradation of the environmental protectiveproperties of the coating. In order for Si-based materials to besuitable for more demanding aircraft engine applications such as vanes,blades and combustors, coatings will be required that exhibit lowerrecession rates.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a protective coating and processes andcompositions used to deposit such a coating, for example, on asilicon-containing article that will be exposed to high temperatures,such as the hostile thermal environment of a gas turbine engine. Theprotective coating has a BSAS composition that is less susceptible todegradation by volatilization as compared to prior art BSAS-basedcoating compositions.

The coating composition of this invention has at least an outer surfaceregion that consists essentially of stoichiometric crystalline phases ofBSAS and is substantially free of a nonstoichiometric second crystallinephase of BSAS. As used herein, “substantially free” means at most tenvolume percent and preferably not more than five volume percent of thesecond phase. The second phase is a lamella phase that has beendetermined by this invention to contain roughly an equamolar ratio ofBaO/SrO, Al₂O₃, and SiO₂, and therefore differs from stoichiometric BSAS(molar ratio: 0.75BaO.0.25SrO.Al₂O₃.2SiO₂; molar percent:18.75BaO.6.25SrO.25Al₂O₃.50SiO₂). As such, the second phase contains asubstoichiometric amount of silica. Furthermore, the second phase tendsto be strontium-rich, i.e., SrO constitutes greater than 25 molarpercent of the combined BaO+SrO content in the second phase. Accordingto one aspect of the invention, the second phase of BSAS has beendetermined to form when stoichiometric BSAS is deposited usingpreviously known (e.g., Eaton et al.) techniques and powdercompositions. While the second phase has been observed and ignored inthe past, the present invention has identified the second phase as beingunstable and volatilizing when subjected to high-temperatureenvironments that contain water and/or corrosive agents such as seasalt. Accordingly, by eliminating the second phase from at least theouter surface region of the protective coating, degradation of theprotective coating is inhibited.

A protective coating of this invention can be produced by carrying outdeposition and heat treatment steps that result in at least the outersurface region of the protective coating consisting essentially ofstoichiometric phases of BSAS and substantially free of the secondphase. According to one embodiment of the invention, the protectivecoating is deposited so that substantially the entire protective coatingconsists essentially of BaO, SrO, Al₂O₃ and SiO₂ in approximatelystoichiometric amounts for BSAS, and after heat treatment the protectivecoating consists essentially of one or more stoichiometric BSAS phasesand is substantially free of the nonstoichiometric silica-lean secondphase. Alternatively, the protective coating can be deposited to containnon-stoichiometric amounts of BaO, SrO, Al₂O₃ and SiO₂ for BSAS so as tocontain the second phase following heat treatment, but then undergoes asecond heat treatment step during which the second phase within at leastthe outer surface region of the coating is volatilized (and resultantporosity is sealed), such that essentially only one or morestoichiometric BSAS phases remain in the outer surface region. With thisapproach, a second region of the protective coating may remain beneaththe outer surface region and contain non-stoichiometric BSAS and thesecond phase.

BSAS protective coatings of this invention are capable of withstandinghigher temperatures and/or longer exposures to the combustionenvironment within a gas turbine engine as a result of the substantialabsence of the second phase, whose susceptibility to volatilizationleads to coating recession. The protective coatings of this inventionare also believed to be more resistant to corrosive environments (e.g.,sea salt) and CMAS-containing environments, which also lead to severedegradation and increased recession rates of BSAS coatings of the priorart. As such, the BSAS protective coatings of this invention are able toexhibit improved performance in corrosive environments and exhibitincreased volatilization resistance at temperatures above 2300° F.(about 1260° C.) while subjected to high gas velocities, enabling thecoatings to be used in more demanding aircraft engine applications suchas vanes, blades and combustors.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a barrier coating system in accordancewith the present invention.

FIG. 2 is a scanned image of a BSAS coating having a crystallinemicrostructure containing the stoichiometric celsian phase of BSAS and anonstoichiometric second phase of BSAS as a result of being deposited inaccordance with prior art practices.

FIGS. 3 and 4 are scanned images of a BSAS coating of the type shown inFIG. 2 following exposure to high temperatures and high gas velocities,resulting in volatilization of the second phase within the coating.

FIGS. 5, 6 and 7 are scanned images of a BSAS coating of the type shownin FIG. 2 following exposure to water vapor and sea salt at hightemperatures, during which the second phase within the coating wasreacted to form volatile species.

FIG. 8 is a graph comparing the atomic silicon contents of thestoichiometric celsian phase of BSAS and the nonstoichiometric secondphase of BSAS.

FIG. 9 is a graph showing the phase stability range for thestoichiometric celsian phase of BSAS as a function of SrO content.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a coating composition for anenvironmental barrier coating system suitable for protectingsilicon-containing components subjected to high temperatures in thepresence of water (water vapor) and corrosive agents, including the highand low pressure turbine vanes (nozzles) and blades (buckets), shrouds,combustor liners and augmentor hardware of gas turbine engines. Examplesof silicon-containing materials for such applications include monolithicmaterials (e.g., silicon carbide, silicon nitride, etc.), compositematerials containing a dispersion of silicon carbide, silicon nitride,and/or silicon reinforcement material in a metallic or nonmetallicmatrix, composite materials having a silicon carbide, silicon nitrideand/or silicon-containing matrix, and composite materials that employsilicon carbide, silicon nitride and/or silicon as both thereinforcement and matrix materials (e.g., SiC/SiC ceramic matrixcomposites (CMC)). While the advantages of this invention will bedescribed with reference to gas turbine engine components, the teachingsof the invention are generally applicable to any silicon-containingcomponent whose silicon content is subject to volatilization.

A multilayer coating system 14 in accordance with an embodiment of thisinvention is schematically represented in FIG. 1. The coating system 14is shown as protecting a surface region 12 of a silicon-containingcomponent 10. The coating system 14 is represented as including aprotective coating 20 formed of BSAS and a multilayer bond coatcomprising bond coat layers 16 and 18. The coating system 14 is intendedto provide environmental protection to the underlying surface region 12and reduce the operating temperature of the component 10, therebyenabling the component 10 to survive within higher temperatureenvironments than otherwise possible. To promote the latter, a top coat(not shown) of a suitable insulating material such as stabilizedzirconia, preferably yttria-stabilized zirconia (YSZ), may be depositedon the outer surface of the protective coating 20.

The major mechanism for degradation of silicon and silicon-basedcompounds (e.g., silicon carbide) in a water-containing environment isthe formation of volatile silicon hydroxide (Si(OH)₄). As known,alkaline-earth metal ceramic compositions such as BSAS exhibit lowdiffusivity to oxidants, e.g., oxygen and water vapor, and therefore areable to inhibit oxidation of the silicon content within the surfaceregion 12, while also being sufficiently chemically and physicallycompatible with the surface region 12 to remain adherent to the region12 under severe thermal conditions. The bond coat layers 16 and 18 serveto promote the adhesion of the coating 20 to the surface region 12 ofthe component 10. Suitable materials for the bond coat layers 16 and 18include silicon and mixtures of mullite and BSAS, respectively, inaccordance with prior art practice. In addition to providingenvironmental and thermal protection to the underlying surface region12, the BSAS protective coating 20 is also physically compliant withSiC-containing materials (such as the surface region 12) and isrelatively compatible with mullite and the Si-based surface region 12 interms of CTE. A suitable thickness range for the protective coating 20is about 50 to about 1000 micrometers, depending on the particularapplication.

According to the invention, the BSAS composition of the protectivecoating 20 has a crystalline microstructure characterized by thepresence of one or more stoichiometric phases (molar ratio:0.75BaO.0.25SrO.Al₂O₃.2SiO₂) of BSAS and, at least within an outersurface region 22 of the coating 20, by the substantial absence of asecond crystalline phase of BSAS that contains approximately equal molaramounts of BaO+SrO, Al₂O₃, and SiO₂. More particularly, the second phasesought to be avoided by the present invention is a lamella phase thatcontains a substoichiometric amount of silica, and furthermore tends tobe strontium-rich, i.e., SrO constitutes greater than 25 molar percentof the combined BaO+SrO content in the second phase. The second phase isintentionally avoided (ideally zero volume percent, preferably not morethan five volume percent, and at most ten volume percent) in at leastthe outer surface region 22 based on investigations discussed below,which showed that the second phase is prone to volatilization whenexposed to water vapors at high temperatures and prone to formingvolatile reaction products when exposed to corrosive agents at hightemperatures.

The following discussion will use the term celsian crystalline phase torefer to the celsian crystallographic structure corresponding tostoichiometric BSAS, and “stoichiometric phase” to refer to both thecelsian and hexacelsian crystalline phases of BSAS, which have the samestoichiometric chemistry. The crystalline celsian phase is the preferredphase, though the crystalline hexacelsian phase may also be present inthe coatings of this invention.

To ensure a crystalline microstructure that consists essentially of thestoichiometric phase, the BSAS composition of the protective coating 20of this invention preferably has a silica content of at least 47 molarpercent, and preferably has a near-stoichiometric silica content, i.e.,about 50 molar percent or more. For this purpose, the protective coating20 may have the stoichiometric composition for BSAS (by molar percent,about 18.75% barium oxide, about 6.25% strontia, about 25% alumina,about 50% silica, and likely incidental impurities) throughout itsthickness or at least in the outer surface region 22. With sufficientsilica content, the protective coating 20 (or at least its outer surfaceregion 22) can be processed in accordance with the invention to besubstantially free of the second phase. This aspect of the invention isevident from FIG. 8, which summarizes data obtained duringinvestigations that led to the invention. FIG. 8 evidences that thestoichiometric celsian crystalline phase of BSAS and thenonstoichiometric second phase of BSAS contain distinctly differentamounts of silicon (and therefore, silica). Whereas the celsiancrystalline phase has a nominal silicon atomic content of about 15.4%,generally encompassing a range of about 14.3 to 17.2 atomic percentbased on investigations discussed below, the second phase appears tohave a silicon atomic content of less than 11.4%. From XRF (X-rayfluorescence) bulk chemistry measurements made on coatings deposited byair plasma spraying (APS) and powders used to deposit the coatings, itwas determined that a powder having a silicon content near thetheoretical 15.4 atomic percent would produce a coating having a bulkaverage silicon content of about 14.4 atomic percent.

In view of the above-noted tendency for strontia to constitute greaterthan 25 molar percent of the BaO+SrO content in the second phase, theprotective coating 20 (or at least its outer surface region 22)preferably has a BaO+SrO content of about 25 molar percent in accordancewith the stoichiometric composition of BSAS, but with strontiaconstituting less than 25 molar percent of the BaO+SrO content, i.e.,the coating 20 has a strontia content of less than 6.25 molar percent.The chemistries of the stoichiometric celsian and nonstoichiometricsecond crystalline phases of BSAS can also be plotted as a function ofstrontia content. From FIG. 9, which again summarizes data obtainedduring investigations that led to the invention, it can be seen that forstrontia contents of about 2.3 to about 6.9 weight percent, the BaO+SrOcontent remains approximately constant. Converting the BaO+SrO contentto atomic percent of Ba+Sr shows the Ba+Sr level to be within the Ba+Srrange for the celsian crystalline phase (0.8 to 1.9 atomic percentstrontium based on a constant total Ba+Sr atomic percent of 7.6 to7.8%). As can be seen from comparing this strontium level to thereported average for stoichiometric BSAS, the coating compositionssummarized in FIG. 9 can be seen to be a strontium-lean celsiancrystalline phase (1.7 to 1.9 atomic percent strontium). Furthercomparison of these levels with those found by XRF of APS-deposited bulkBSAS coatings (average Ba+Sr of about 8.4 atomic percent) evidences thatthe coating materials had Ba+Sr levels greater than the maximumstrontium solubility in the celsian crystalline phase seen in FIG. 9. Itwas theorized that due to the excess strontium (from microprobe results,an average of 2.2 atomic percent compared with 1.7 to 1.9 atomicpercent), the BSAS coatings were forced to form the strontium-richsecond phase. As suggested from FIG. 9, the amount of the second phaseformed would be expected to depend on the amount of strontium in thebulk material.

Based on the above observations and analysis, it is believed thatprevious attempts to produce a stoichiometric BSAS coating from powderscontaining near-stoichiometric amounts of barium oxide, strontium,alumina and silica have resulted in the formation of the second phasedescribed herein, in addition to the desired stoichiometric celsiancrystalline phase of BSAS. In view of the above analysis andinvestigations discussed below, it was concluded that the second phasecan be avoided or minimized by adjusting the silica content of thepowder to compensate for silica losses during deposition. Alternativelyor in addition, it is believed that the second phase can be avoided orminimized by reducing the strontia content in the powder to suppress theformation of the second phase on the basis of strontium solubility inthe celsian crystalline phase. The latter approach would require thepowder to have a silica content at or above the stoichiometric amount(50 molar percent) for BSAS, and slight compensation with the aluminacontent in the powder. On this basis, suitable approximate compositions(molar percent) for powders that can be used to deposit by APS a BSAScoating that is substantially free of the second phase are summarizedbelow.

Powder A Powder B BaO 18.7-19.1% 18.4-18.8% SrO  4.5-4.9%  4.5-4.9%Al₂O₃ 25.1-26.1% 23.4-24.4% SiO₂ 50.4-51.4% 52.3-53.3%

The composition of Powder A is based on reducing the strontia content ofthe powder per the strontium solubility model while providing a silicacontent of at least 50 molar percent. The composition of Powder B isalso based on lowering the strontia content, but with a further increasein the silica content of the powder. In each case, the silica contenthas been increased above that used in the past, and compensates forsilica volatilization that occurs during deposition by APS. Furthermore,strontia constitutes less than 25 molar percent of the BaO+SrO contentof each powder, and therefore BSAS coatings deposited from the powders.Comparing Powders A and B, it can be seen that their BaO+SrO contentsare substantially the same and below the 25 molar percent level ofstoichiometric BSAS, yielding a silica to BaO+SrO molar ratio of greaterthan 2:1. A notable difference is that the alumina content of Powder Bis lower than in Powder A to accommodate the higher silica content inPowder B. Powder B would be predicted to produce an as-deposited coatingwhose bulk chemistry is substantially stoichiometric. In contrast, thehigher alumina content of Powder A (greater than 25 molar percent)pushes the composition of the resulting BSAS coating toward a morealumina-rich composition. As a result of silica loss during deposition,BSAS coatings deposited from Powder A should fall near thecelsian-alumina two-phase region, resulting in an alumina phase contentof about 2 atomic percent. This additional phase is believed to providean added margin for avoiding the strontia-rich second phase. Because analumina phase content in excess of 20 atomic percent may affect thermalexpansion to a significant degree, a suitable upper limit for thealumina phase content in BSAS coatings deposited from Powder A isbelieved to be about 20 atomic percent.

The above observations and analysis were based on a series ofinvestigations that led to this invention. In a first investigation,BSAS coatings intended to have the stoichiometric composition weredeposited on Si-containing substrates by APS followed by heat treatmentin accordance with U.S. Pat. No. 6,410,148. The powder used in thedeposition process had a near-stoichiometric composition of, by molarpercent, about 19.0% barium oxide, about 6.8% strontia, about 24.8%alumina, and about 49.4% silica, and therefore had a silica content onlyslightly below the stoichiometric amount of 50 molar percent. Duringdeposition, the substrates were held at about 250° C. to about 350° C.Following deposition, the coatings were heat treated at about 1250° C.for about twenty-four hours to provide stress relief and promote bondingof the sprayed particles.

An SEM image of one of the coatings is shown in FIG. 2, and evidencesthat the coating contained two crystalline phases: the celsiancrystalline phase corresponding to stoichiometric BSAS, and thenonstoichiometric second phase discussed above, which is iso-structuralto the celsian crystalline phase (e.g., exhibits the same X-raypattern). In FIG. 2, the celsian crystalline phase appears as the darkerregions while the lighter regions are the second phase. The second phasewas determined to be nonstoichiometric, having higher SrO₂ and Al₂O₃contents and a lower SiO₂ content than the stoichiometric celsiancrystalline phase.

The coatings were exposed for about fifty hours to temperatures ofbetween about 2500 and 2900° F. (about 1370 and 1600° C.) andwater-containing air at high velocities of about Mach 0.3 to 0.5. SEMimages of two of the coatings are shown in FIGS. 3 and 4. On carefulexamination of the coatings, it was determined that portions of thestrontia-rich second phase had volatized, whereas the stoichiometriccelsian crystalline phase had remained very stable. It was concludedthat when exposed to water at high gas velocities, the second phasebecame unstable and began to volatilize through the loss of silica,resulting in pore formation throughout the coating thickness. In theas-deposited condition (e.g., deposition by APS and then heattreatment), the celsian and second crystalline phases of BSAS coatingsappeared to be in equilibrium with each other. However, it was concludedthat as the strontia-rich second phase degraded during thehigh-temperature exposure to water vapor, the chemical composition ofthe second phase changed, resulting in the second phase reacting withthe celsian (stoichiometric) crystalline phase of BSAS to formintermediate reaction products that are also volatile. The end resultwas the formation of through-thickness porosity at the originallocations of the second phase and the subsequent formation ofintermediate volatile phases, all of which led to the degradation of thecoatings as seen in FIGS. 3 and 4.

In another investigation, essentially identical BSAS coatings wereexposed for about fifty hours to a combination of water vapor and seasalt (0.5-1 ppm) at temperatures between about 2200 and 2500° F. (about1200 and 1370° C.). In FIGS. 5, 6 and 7, which are SEM images of one ofthe coatings, pores can be seen within the coating. Analysis of thecoating indicated that GaO and MgO components of the sea salt hadinterdiffused into the strontia-rich second phase of the coating,forming MgO and GaO-silicate type amorphous components that were highlyvolatile. While there was no evidence that the sea salt species haddiffused into the celsian crystalline phase, the MgO and GaO-silicatetype amorphous components appeared to have quickly reacted with thecelsian crystalline phase of the coating, forming additional amorphousvolatile components and additional porosity.

From these investigations, it was concluded that a BSAS coatingconsisting of closer to 100% of the stoichiometric celsian phase of BSASwould be much less volatile and more corrosion resistant underconditions within a gas turbine engine. However, it was furtherconcluded that additional steps were necessary to avoid formation of thesecond phase and thereby produce an as-deposited coating structure thatis substantially free of the second phase, or to eliminate the secondphase following coating deposition. In addition to tailoring thecomposition of the sprayed powder as discussed above, such steps mightinclude process modifications and/or pre-processing of the powder (e.g.,heat treating) before the spraying operation. Notably, it is theorizedthat the entire BSAS coating need not be substantially free of thesecond phase, in that sufficient improvement in coating performanceshould be achieved if just the outer surface region were processed to besubstantially free of the second phase.

In a third investigation, additional BSAS coatings were deposited by APSon CFCC (continuous fiber ceramic composite) combustor liners for anextended engine test. As with the first and second investigations, thepowder material used to deposit the coatings had a composition of, inmolar percent, about 19.0% BaO, about 6.8% SrO, about 24.8% Al₂O₃, andabout 49.4% SiO₂, and therefore had a silica content only slightly belowthe stoichiometric amount. Also consistent with the two previousinvestigations, the coatings were targeted to have stoichiometriccompositions (in molar percent, 18.75% BaO, 6.25% SrO, 25% Al₂O₃, and50% SiO₂). However, XRF showed the coatings to have as-depositedcompositions of, in molar percent, about 20.1% BaO, about 7.2% SrO,about 26.0% Al₂O₃, and about 46.8% SiO₂. Therefore, the coatings had asub-stoichiometric silica content (i.e., below 50 molar percent), andstrontia constituted more than 26 molar percent of the BaO+SrO content.A comparison of the powder and as-deposited coating chemistriesindicated that the APS process caused a nominal 7.4 weight percent lossof SiO₂ by volatilization. SEM imaging of the coatings showed atwo-phase crystalline structure similar to that seen in FIG. 2, namely,the stoichiometric celsian phase and the non-stoichiometric secondphase. Image analysis approximated the second phase as present in anamount of about 15 volume percent.

It is worth noting that, because essentially the same powdercompositions and spray processes were used in all three investigations,the BSAS coatings produced in the first and second investigationspresumably had essentially the same chemistries as that found for thecoatings produced in the third investigation, including asub-stoichiometric silica content of less than 47 molar percent and aBaO+SrO content of which SrO constitutes more than 25 molar percent.

From the above, it was concluded that the combination ofnear-stoichiometric BSAS powders and deposition processes used todeposit BSAS coatings in the past (e.g., Eaton et al.) were the cause ofthe presence of the second phase. In contrast to conventional wisdom,the conclusion drawn by this invention is that the second phase isundesirable as a result of being prone to volatilizing in water and gasvelocity environments and causing coating degradation in sea-saltcontaining environments, both of which result in the formation of poresin BSAS coatings. Accordingly, the BSAS protective coatings of thepresent invention are deposited to have chemistries that inhibit theformation of the second phase, namely, through having a silica contentof at least 47 molar percent and preferably about 50 molar percent, andpreferably through having a BaO+SrO content of about 25 molar percentbut with strontia constituting less than 25 molar percent of the BaO+SrOcontent, i.e., a strontia content of less than 6.25 molar percent. Suchcoatings are capable of being substantially free of pores formed byvolatilization after extended periods at elevated temperatures, such asduring operation of a gas turbine engine in which a CMC componentprotected by the coating is installed.

As was discussed above, the more optimal BSAS chemistry proposed by thisinvention can be obtained by employing a powder whose chemistry issufficiently rich in silica so that silica losses due to volatilizationduring deposition result in the as-deposited coating have a silicacontent of at least 47 molar percent and preferably at or near 50 molarpercent. As an alternative or in addition to an increased silicacontent, a powder can be used whose chemistry is sufficiently lean instrontia to inhibit the formation of the second phase as a result of thestrontium solubility in the celsian crystalline phase. Immediatelyfollowing deposition by APS, such compositions should produce a coatinghaving a composition that would ensure a celsian crystalline phasecontent of very near 100%.

On the basis of the investigations described above, an alternativemethod to developing a near 100% celsian crystalline phase in a BSAScoating is to use a stoichiometric or near-stoichiometric BSAS powder inaccordance with prior practices, with the result that the coating wouldcontain a sub-stoichiometric amount of silica as a result ofvolatilization. Following heat treatment, after which both the celsianand second crystalline phases would be present, the coating is subjectedto a temperature sufficient to eliminate the second phase by intentionalvolatilization. A suitable heat treatment for this purpose is believedto be near but below the melting temperature (1414° C.) of free siliconwithin the substrate or coating system. If a furnace is used, theduration of the heat treatment can be adjusted to control the amount ofsecond phase volatized and to seal the resultant porosity. For example,a duration of about two to about ten hours is believed to be sufficientto eliminate the second phase from about 0.5 to 1.0 mil (about 10 toabout 25 micrometers) of the outermost surface region of the coating,with longer durations increasing the depth to which the second phase iseliminated from the coating. With reference to FIG. 1, in such anapproach the region 24 of the coating 20 beneath the outer surfaceregion 22 would still contain non-stoichiometricbarium-strontium-aluminosilicate (e.g., less than 47 molar percentsilica), and therefore contain the second phase in addition tostoichiometric phases. In addition to heat treatments performed within afurnace, surface heat treatments can be performed by such methods aslaser glazing or in-situ surface treatments performed with the plasmaspray gun after coating deposition and with the powder feed turned off.An advantage of performing a surface treatment is the ability to locallyheat the coating above the melting temperature of free silicon in thesubstrate beneath the coating.

While the invention has been described in terms of a preferredembodiment, it is apparent that other forms could be adopted by oneskilled in the art. For example, suitable powder compositions willdepend on the deposition process, which include thermal spray processesin addition to APS. Therefore, the scope of the invention is to belimited only by the following claims.

1. A process of forming a protective coating on a silicon-containingsurface, the protective coating consisting essentially of barium oxide,strontia, alumina, and silica, and incidental impurities so as to have abarium-strontium aluminosilicate composition, the protective coatinghaving at least an outer surface region that consists essentially of oneor more stoichiometric crystalline phases of barium-strontiumaluminosilicate of which at least one is a celsian crystalline phase, atleast the outer surface region being substantially free of anonstoichiometric and silica-lean second crystalline phase ofbarium-strontium aluminosilicate, the process comprising: depositing theprotective coating by spraying a powder that contains non-stoichiometricamounts of barium oxide, strontia, alumina, and silica, wherein silicaconstitutes more than 50 molar percent of the powder and/or strontiaconstitutes less than 6.25 molar percent of the powder, and whereinsilica volatilizes during the spraying of the powder so that at leastthe outer surface region of the protective coating consists essentiallyof BaO, SrO, Al2O3 and SiO2 in approximately stoichiometric amounts forbarium-strontium aluminosilicate; and then heat treating the protectivecoating so that at least the outer surface region of the protectivecoating consists essentially of the one or more stoichiometriccrystalline phases of barium-strontium aluminosilicate and issubstantially free of the nonstoichiometric and silica-lean secondcrystalline phase.
 2. A process according to claim 1, wherein the powdercontains more than 50 molar percent silica.
 3. A process according toclaim 1, wherein the powder contains at least 52 molar percent silica.4. A process according to claim 1, wherein the powder has a silica toBaO+SrO molar ratio of greater than 2:1 and an alumina content ofgreater than 25 molar percent.
 5. A process according to claim 1,wherein the protective coating is deposited so that substantially theentire protective coating consists essentially of BaO, SrO, Al₂O₃ andSiO₂ in approximately stoichiometric amounts for barium-strontiumaluminosilicate, and after the first heat treatment step the protectivecoating consists essentially of the celsian crystalline phase ofbarium-strontium aluminosilicate and is substantially free of thenonstoichiometric and silica-lean second crystalline phase ofbarium-strontium aluminosilicate.
 6. A process according to claim 5,wherein the powder contains more than 50 molar percent silica.
 7. Aprocess according to claim 6, wherein the powder consists essentiallyof, by molar percent, 18.7-19.1% BaO, 4.5-4.9% SrO, 25.1-26.1% Al₂O₃,and 50.4-51.4% SiO₂.
 8. A process according to claim 5, wherein thepowder contains at least 52 molar percent silica.
 9. A process accordingto claim 8, wherein the powder consists essentially of, by molarpercent, 18.4-18.8% BaO, 4.5-4.9% SrO, 23.4-24.4% Al₂O₃, and 52.3-53.3%SiO₂.
 10. A process according to claim 5, wherein the protective coatingconsists of, by molar percent, about 25% barium oxide+strontia, about25% alumina, about 50% silica, and incidental impurities.
 11. A processaccording to claim 10, wherein strontia constitutes less than 25 molarpercent of the barium oxide+strontia content of the protective coating.12. A process of forming a protective coating on a silicon-containingsurface, the protective coating consisting essentially of barium oxide,strontia, alumina, and silica, and incidental impurities so as to have abarium-strontium aluminosilicate composition, the protective coatinghaving at least an outer surface region that consists essentially of oneor more stoichiometric crystalline phases of barium-strontiumaluminosilicate of which at least one is a celsian crystalline phase,and at least the outer surface region is substantially free of anonstoichiometric and silica-lean second crystalline phase ofbarium-strontium aluminosilicate, the process comprising: depositing theprotective coating by spraying a powder that contains stoichiometricamounts of barium oxide, strontia, alumina, and silica, wherein silicavolatilizes during the spraying of the powder so that the protectivecoating as deposited contains non-stoichiometric amounts of BaO, SrO,Al₂O₃ and SiO₂ for barium-strontium aluminosilicate; performing a firstheat treatment step so that the protective coating contains thenonstoichiometric and silica-lean second crystalline phase; and thenperforming a second heat treatment step during which the outer surfaceregion forms by volatilization of the nonstoichiometric and silica-leansecond crystalline phase within the outer surface region, wherein atleast the outer surface region consists essentially of the one or morestoichiometric crystalline phases of barium-strontium aluminosilicateand is substantially free of the nonstoichiometric and silica-leansecond crystalline phase, and resultant porosity is sealed within theouter surface region.
 13. A process according to claim 12, wherein theprotective coating has a second region beneath the outer surface region,the second region containing non-stoichiometric barium-strontiumaluminosilicate and the nonstoichiometric and silica-lean secondcrystalline phase.
 14. A process according to claim 13, wherein theouter surface region of the protective coating contains at least 47molar percent silica and the second region of the protective coatingcontains less than 47 molar percent silica.
 15. A process according toclaim 12, wherein the second heat treatment step is a localized surfaceheat treatment of the outer surface region.
 16. A process according toclaim 1, wherein the protective coating is deposited as part of abarrier coating system on the silicon-containing surface, the processfurther comprising the step of depositing at least one intermediatelayer on the silicon-containing surface after which the protectivecoating is deposited on the at least one intermediate layer, the atleast one intermediate layer containing a material chosen from the groupconsisting of silicon and mullite.
 17. A process of forming a protectivecoating on a silicon-containing surface of a gas turbine enginecomponent, the protective coating being a part of a barrier coatingsystem comprising at least one intermediate layer on which theprotective coating is deposited, the process comprising the steps of:depositing the protective coating so that substantially the entireprotective coating consists essentially of BaO, SrO, Al₂O₃ and SiO₂ inapproximately stoichiometric amounts for barium-strontiumaluminosilicate, the protective coating being deposited by spraying apowder that contains non-stoichiometric amounts of barium oxide,strontia, alumina, and silica, wherein silica constitutes more than 50molar percent of the powder and/or strontia constitutes less than 6.25molar percent of the powder; and heat treating the protective coating sothat the protective coating consists essentially of a celsiancrystalline phase of barium-strontium aluminosilicate and not more thanfive volume percent of a nonstoichiometric crystalline lamella phase ofbarium-strontium aluminosilicate that contains a substoichiometricamount of silica.
 18. A process according to claim 17, wherein theprotective coating contains at least 50 molar percent silica followingthe depositing step.
 19. A process according to claim 17, wherein afterthe depositing step the protective coating consists of, by molarpercent, about 25% barium oxide+strontia, about 25% alumina, about 50%silica, and incidental impurities.
 20. A process according to claim 19,wherein strontia constitutes less than 25 molar percent of the bariumoxide+strontia content of the protective coating after the depositingstep.
 21. A process according to claim 17, wherein the protectivecoating is deposited by spraying a powder consisting essentially of, bymolar percent, 18.7-19.1% BaO, 4.5-4.9% SrO, 25.1-26.1% Al₂O₃, and50.4-51.4% SiO₂.
 22. A process according to claim 17, wherein theprotective coating is deposited by spraying a powder consistingessentially of, by molar percent, 18.4-18.8% BaO, 4.5-4.9% SrO,23.4-24.4% Al₂O₃, and 52.3-53.3% SiO₂.
 23. A process according to claim17, further comprising the steps of installing the component in a gasturbine engine and conducting an engine test, after which the protectivecoating is substantially free of pores formed by volatilization of theprotective coating.